Power control is important for capacity and efficiency in code division multiple access (CDMA) communication systems. For example, if a mobile terminal is located close to a base station, the power level of signals transmitted from the mobile terminal to the base station, in the absence of power adjustment, would be comparatively high. This could interfere with transmissions from other mobile terminals located farther from the base station. Conversely, the power level of signals transmitted from a mobile terminal which is located far from the base station, in the absence of power adjustment, could be comparatively weak. Accordingly, some communication systems, including, for example, wideband CDMA (WCDMA) systems, provide a transmission power control (TPC) command by which a base station can adjust the transmission power of a mobile terminal.
Power control can involve the base station estimating the signal-to-interference ratios (SIRs) of received signals, such as, for example, received pilot symbols in a CDMA system. If the SIRs of a received signal are lower than a threshold value, a command or adjustment signal is sent to the transmitting mobile terminal to increase its transmission power. The command is sent on the reverse link, i.e. the down link. When the estimated SIRs are higher than the threshold value, a command to decrease transmission power is sent by the base station to the transmitting mobile terminal. Base station transmission power may also similarly be controlled by the mobile terminal.
In some communication systems, such as 3rd Generation Partnership Project (3GPP) standard communications systems, a mobile terminal should stop transmitting when the quality of the signal received from a transmitting base station falls below a threshold synchronization value, which may indicate that the mobile terminal has become out-of-sync with the transmitting base station. Accordingly, detection of when the mobile terminal is in-sync or out-of-sync with a transmitting base station can be important to avoid unnecessarily dropping calls, or conversely, to avoid continuing to transmit and possibly degrade the capacity of the base station after the mobile station can no longer effectively communicate with the base station.
According to the 3GPP standard, a mobile terminal is considered to have become out-of-sync, and therefore should stop transmitting, when the Dedicated Physical Channel Signal to Interference Ratios (DPCH-SIR), averaged over 160 milliseconds, is below a threshold value. The 3GPP standard also provides that a mobile terminal is considered to be in-sync, and may transmit, when the DPCH-SIR, averaged over 160 milliseconds, is above a threshold value. A further explanation of this requirement is provided by the 3rd Generation Partnership Project; Physical Layer Procedures (FDD), 3GPP TS 25.101, chapter 6.4.4, (2002–06), 2002, and by the 3rd Generation Partnership Project; Physical Layer Procedures (FDD), 3GPP TS 25.214, chapter 4.3.1.2, (2002–06), 2002.